During semiconductor manufacturing processes, it is a very important stage that a target substrate coated with photoresist is subjected to photolithography which employs a mask plate (reticle) to form a photolithography pattern and an etching process is conducted with the photolithography pattern so as to transfer a pattern on the mask plate onto the target substrate.
The photolithography process is such a process that masking exposure is conducted on a target substrate coated with photoresist through an exposure machine and a mask plate with a slit-like light-transparent region. Eventually, a pattern on the mask plate formed by the slit-like light-transparent region is transferred onto the target substrate, and the target substrate is etched, so as to obtain a slit-like pattern structure with a certain width on the target substrate.
In the field of microlithography, pattern structures drawn on target substrates are mostly linear, such as electrical circuit lines. As the semiconductor devices advanced to miniaturization gradually, minimization of the line width of patterns becomes a goal that manufactures are pursuing. However, owing to restriction upon the exposure precision of an exposure machine, the width of a pattern with a minimum width on a target substrate that has been produced at present is over 3 μm (more than 3 μm), and the line width of a corresponding slit-like light-transparent region on a mask plate is 3 to 5 μm.
For display products with smaller sizes, such as cell phones, reduction of the distance between electrical circuits or decrease of the width of electrical circuit lines is very important for increasing the resolution of products or the aperture ratio of pixels.
Implementing patterns with line widths in the order of micrometer is very common in the field of semiconductor. For example, the distance (a gap width) between the source electrode and the drain electrode of a thin film transistor (TFT) needs to be designed at the order of micron, or the line width of certain electrical circuit lines needs to be designed at the order of micron. Accordingly, the distance between the two edges of a slit-like light-transparent region on a mask plate needs to be designed at the order of micron as well. However, owing to restriction upon the exposure precision of an exposure machine, with an existing exposing system and a corresponding mask plate, on the premise that the photolithographic quality can be guaranteed, only patterns with line widths over 3 μm on a target substrate can be produced, and patterns with line widths of 3 μm or less cannot be obtained. By taking implementation of a gap between a source electrode and a drain electrode of a TFT as an example, the width of the gap between the source electrode and the drain electrode is as small as possible, so as to facilitate increase of the aperture ratio of pixels, and then, optical characteristics of the TFT is improved.
A mask plate for implementing patterns with a line width over 3 μm available currently is illustrated in FIG. 1, and the mask plate comprises a slit-like light-transparent region and lightproof regions, with the edges of the slit-like light-transparent region being arranged to have a linear structure. The vertical distance between the two edges of the slit-like light-transparent region as illustrated in FIG. 1 is 3 μm, and the resultant line width of a gap is no less than 3 μm. If a gap with a narrower line width is desired and the exposure light source of the exposure machine is not changed, what can be done is to decrease the width of the slit-like light-transparent region on the mask plate. However, a linear pattern on a target substrate is a diffraction stripe pattern resulting from diffraction of light emitted by the exposure machine, the diffraction occurs upon the light passing through the slit-like light-transparent region on the mask plate, and the diffraction stripe pattern is a linear pattern. The conditions for light diffraction comprise that the width of a gap (the width of a slit-like light-transparent region) is comparable to or almost the same as the wavelength of incident light (light emitted by an exposure machine). When the gap is too small, a scattering phenomenon of light will happen when the light passes through the gap, and no diffraction pattern can be formed. Even if a diffraction pattern can be formed, the conductive film layer between a source electrode and a drain electrode cannot be etched away completely due to the fact that light intensity received by photoresist on the target substrate is too small and this leads to an incomplete removal of photoresist for the pattern, thereby causing a short circuit between the source electrode and the drain electrode. Therefore, a pattern with a narrower line width cannot be implemented with a conventional mask plate.